There has been spread of a various types of optical discs, such as a DVD and a CD (Compact Disc). The DVD includes a DVD-ROM (DVD-Read Only Memory) for read only, a DVD-R (DVD-Recordable) of a write-once type, a DVD-RW (DVD-ReWritable) of a rewritable type, a DVD-RAM (DVD-Random Access Memory) of a rewritable type, and the like. Moreover, there are also a dual-layer DVD-ROM and a dual-layer DVD-R having two recording layers. Moreover, the CD also includes a CD-ROM for read-only and for computers, a CD-DA (Compact Disc Digital Audio) for read only and for music, a write-once type CD-R, a rewritable type CD-RW, and the like.
Moreover, there has been spread of a disc drive which is integrally provided with a function for information recording or reading with respect to a plurality types of optical discs. Such a disc drive is referred to as a multidisc drive or a multi drive. For example, there has been spread of a multidisc drive for DVD/CD, which is integrally provided with a function for the information recording or reading with respect to almost all of the above-listed optical discs.
Most of the multidisc drives are provided with an optical pickup which is integrally provided with a structure for information recording or reading with respect to a plurality types of optical discs. For example, most of the multidisc drives for DVD/CD are provided with an optical pickup for DVD/CD. As this type of optical pickup, there is listed an optical pickup provided with a structure for focusing a plurality of light beams on an optical disc by using one objective lens. That is, this type of optical pickup is provided with a plurality of light sources corresponding to the types of the optical discs, converts the optical axes of the light beams outputted from the respective light sources to a unified optical axis, and focuses the light beams on the optical disc by using one objective lens which is disposed to cross the unified optical axis.
By the way, the DVD has a higher recording density than the CD. In order to ensure stability or accuracy in the information recording or reading, it is desirable to correct spherical aberration, coma aberration, or the like of the light beam for DVD in the optical pickup. Some of the optical pickups for DVD/CD which are currently popular are provided with an aberration correcting apparatus which uses a liquid crystal (LC) to correct the spherical aberration, the coma aberration or the like of the light beam for DVD. The aberration correcting apparatus is provided, for example, with a LC optical element and a LC driver for driving the LC optical element. The LC optical element is normally located on the incident side of the objective lens and is disposed to cross the unified optical axis. The LC optical element has two electrode layers which are arranged to face each other with the LC therebetween. By applying voltages to the electrodes formed in the electrode layers, it is possible to change LC orientation, LC refractive index, and an optical path length of the light beam which passes through the LC. By this, it is possible to uniform the wavefront of the light beam for DVD and to correct the spherical aberration, the coma aberration, or the like of the light beam for DVD.
On the other hand, the CD has a lower recording density than the DVD, so it has been generally considered that it is only necessary to correct the coma aberration of the light beam for CD. In fact, the aberration correcting apparatus provided for the optical pickups for DVD/CD which are currently popular has only a function for correcting the coma aberration of the light beam for CD.
However, the widespread CD has allowed the presence of the CD with a variety of thicknesses of a protection layer on the side that the light beam is irradiated. In view of the present situation, it is also desirable to correct the spherical aberration of the light beam for CD.
Moreover, there tend to be more types of optical discs which record or read the information by using one optical pickup, and this complicates the optical system of the optical pickup. Furthermore, it is also required to make the optical system of the optical pickup highly accurate in order to realize the information recording or the like with respect to the dual-layer DVD-R. In view of the present situation, it is desirable to correct astigmatism aberration at least for the light beam for DVD.
In order to respond the aforementioned demands, it is ideally desirable to provide one aberration correcting apparatus with a function that corrects each of the spherical aberration, the coma aberration, and the astigmatism aberration for each of the plurality of light beams corresponding to the plurality types of optical discs. This, however, has the following problem.
That is, if the spherical aberration, the coma aberration, and the astigmatism aberration are corrected with respect to one light beam, the amount of a change in the optical path length of the light beam is partially increased. For example, if the spherical aberration, the coma aberration, and the astigmatism aberration are corrected for a certain light beam, there possibly arises the situation that a certain part of the optical path length of the light beam is changed triply in the same direction. In this case, the portion which is changed triply in the light beam has a considerably large amount of change in the optical path length, compared with the other portion.
The LC optical element, however, which is used for the aberration correcting apparatus, has a limited range in which the optical path length of the light beam can be partially changed (hereinafter referred to as a “optical-path-length changeable range”). Thus, in some cases, all of the spherical aberration, the coma aberration, and the astigmatism aberration cannot be corrected completely for one light beam. For example, if a certain part of the optical path length of a certain light beam is firstly changed in order to correct the spherical aberration and the same part of the optical path length of the light beam is secondly changed in the same direction in order to correct the coma aberration, the actual amount of the change in the optical path length of the light beam reaches the limit of the optical-path-length changeable range at this stage, so that there possibly arises the situation that the astigmatism aberration, which is scheduled to be performed next, cannot be corrected.
Moreover, the importance of the aberration correction varies depending on the type of the aberration, such as the spherical aberration, the coma aberration, and the astigmatism aberration. Furthermore, the order of the importance of the aberration correction varies depending on the type of the optical disc. For example, the descending order of the importance of the correction aberrations regarding the DVD-RAM is the astigmatism aberration, the spherical aberration, and the coma aberration. The descending order of the importance of the correction aberrations regarding the dual-layer DVD-R is the spherical aberration, the coma aberration, and the astigmatism aberration. Moreover, in the case of the CD-R, it is perceived that the spherical aberration needs to be corrected as described above; however, the coma aberration is most important in the aberration correction. In this case, if the aberration is corrected in the order of the spherical aberration, the coma aberration, and the astigmatism aberration, there is a possibility that the astigmatism aberration, which is most important for the DVD-RAM, cannot be sufficiently corrected. On the other hand, in order to avoid such a disadvantage, if the aberration is corrected in the order of the astigmatism aberration, the coma aberration, and the spherical aberration, there is a possibility that the spherical aberration, which is most important for the dual-layer DVD-R, cannot be sufficiently corrected. On the other hand, in order to avoid these disadvantages, if the aberration is corrected in the order of the spherical aberration, the astigmatism aberration, and the coma aberration, there is a possibility that the coma aberration, which is most important for the CD-R, cannot be sufficiently corrected.
As described above, since the LC optical element used for the aberration correcting apparatus has the limited optical-path-length changeable range, it is difficult to provide one aberration correcting apparatus with the function that corrects each of the spherical aberration, the coma aberration, and the astigmatism aberration for each of the plurality of light beams corresponding to the plurality types of optical discs.
Incidentally, in order to overcome the difficulty, it is considered to expand the optical-path-length changeable range in the LC optical element used for the aberration correcting apparatus. The expansion of the optical-path-length changeable range can be realized by thickening the LC of the LC optical element.
However, thickening the LC slows down the response of the LC, which increases a time length for the aberration correction. At least the spherical aberration and the astigmatism aberration need to be corrected before the information recording or reading is started with respect to the optical disc. Thus, if the time length for the aberration correction is increased, there is such a problem that it takes a longer time to start the information recording or reproduction after the optical disc is inserted in the disc drive. Therefore, it is not desirable to expand the optical-path-length changeable range by thickening the LC.
The aforementioned problem can also occur on the aberration correcting apparatus which uses not the LC optical element but another optical element.
In order to solve the above-mentioned problems, it is therefore a first object of the present invention to provide an aberration correcting apparatus, an optical pickup and a aberration correcting method, which allow effective aberration correction for a plurality of light beams corresponding to a plurality types of optical discs, to thereby ensure stability or appropriate accuracy in information recording or reading with respect to the plurality types of optical discs.
It is a second object of the present invention to provide an aberration correcting apparatus, an optical pickup and an aberration correcting method, which can perform the aberration correction on a plurality of light beams corresponding to a plurality types of optical discs, in accordance with the importance, without expanding the optical-path-length changeable range of an optical element which performs the aberration correction.